Process for producing permeation resistant containers

ABSTRACT

The present invention discloses an improved process for producing plastic containers with excellent resistance to permeation by solvents such as hydrocarbons, hydrocarbon fuels, and hydrocarbon fuels with organic additives including lower alkanols and ethers consistently and reliably. The process is related to a multi-step treatment of plastic containers in a controlled manner with fluorine containing gases while blow molding them. In the process of the present invention, a parison is formed from a pre-heated thermoplastic material, expanded within a closed mold by means of an inflating gas for conforming the parison to the shape of the mold, and subjected to multiple fluorination treatment steps to effect fluorination of the interior surface of the parison. In the first step of the multi-step fluorination treatment the parison is pressurized with a reactive gas containing from about 0.05 to about 0.54% fluorine by volume while the parison is at a temperature above its self supporting temperature and for a time sufficient to effect fluorination of the interior surface of the parison. Subsequently, the interior surface of the pre-fluorinated parison is pressurized with a reactive gas containing fluorine in a concentration that is greater than that used in the initial fluorination step for a time sufficient to form said fluorinated parison with excellent resistance to solvent permeation. The parison is purged with an inert gas, vented and the container recovered. The key requirements for producing containers with excellent resistance to solvent permeation consistently and reliably include (1) minimizing and/or eliminating the molecular oxygen contaminant in the parison while treating it with fluorine containing reactive gas and (2) selecting the concentration of fluorine in the reactive gas for the first and subsequent fluorination treatment steps in such a way that it eliminates both under and over fluorination of the inside surface of the parison.

FIELD OF THE INVENTION

This invention relates to a process for producing permeation resistantcontainers.

BACKGROUND OF THE INVENTION

Fluorination of polyethylene and other polymeric materials to improvetheir resistance to solvents and to vapor permeation has long beenpracticed. Early work was reported by Joffre in U.S. Pat. No. 2,811,468and Dixon et al. in U.S. Pat. No. 3,862,284. The '468 patent discloses aprocess of fluorinating polyethylene material in a chamber to improvebarrier properties thus enhancing the material as a wrapping materialfor foodstuffs and perishable materials. More specifically, it disclosesa fluorination process carried out at room temperature or at atemperature below about 50° C. with 100% fluorine or an inert gascontaining 10% fluorine for a period of about 20 to 150 minutes toachieve surface fluorine concentrations of 0.03 to 3.5 percent by weightof fluorine based on the weight of the polyethylene. Dixon et al., in'284 discloses fluorination of a variety of polymeric materials in blowmolding operations to enhance their barrier properties. A treatment gascontaining from about 0.1 to 10% by volume of fluorine in an inert gaswas injected into the parison to inflate or expand it into shape. Due tothe higher temperature, a combined blowing and reaction time ofapproximately 5 seconds was utilized at which time the parison wascooled and the reactive gas and container recovered.

Commercially fuel tanks having enhanced resistance to hydrocarbonpermeation have been marketed under the Airopak trademark wherein thefuel tanks are produced by utilizing blow molding techniques. In theseprocesses the parison is initially conformed to the desired shape byinflating or expanding with an inert gas, followed by purging of theparison and subsequent injection of the parison with a reactive gascontaining from 0.1 to 104 fluorine. The reactive gas is removed fromthe parison, recovered and the container ejected from the mold.

There have been substantial modifications to the early processes for theproduction of containers having enhanced barrier properties via blowmolding. Some of the processes are described in the following patents:

U.S. Pat. No. 4,142,032 discloses an apparent improvement in the Dixonet al., process utilizing a reactive gas containing both fluorine andbromine at temperatures below the softening point of the polymer andpressures one atmosphere or less. Basically, then, the '032 process issimilar to that of Joffre '468 in that the fluorination/bromination iseffected at low temperature, thus requiring long reaction times.

U.S. Pat. No. 4,404,256; 4,264,750; and 4,593,050 disclose a lowtemperature fluorination of polyolefins, e.g., polyethylene andpolypropylene, to form low energy surfaces utilizing wave energy inassociation with the fluorination process. The. '256 and '750 patentsdisclose contacting the polymer surface with ions or radicals comprisingfluorine or fluorinated carbon in a cold plasma. The '050 disclosesfluorination of polymer surfaces utilizing a fluorinating gas andenhancing the fluorination by exposing the surface to ultravioletradiation to assist in the fluorination process.

U.S. Pat. No. 4,701,290 discloses the production of high densitypolyethylene fuel tanks having increased barrier resistance tohydrocarbon solvent and vapor permeation via off-line fluorination. Thekey to enhancing barrier permeation resistance lies in the precisecontrol of fluorination of the polyethylene fuel tank and this isachieved by passing the treatment gas through a container filled withaluminum oxide. By measuring the quantity of oxygen generated from theAl₂ O₃, one controls the concentration of fluorine contained in thetreatment gas and thereby controls the level of fluorine acting upon thesurface of the container within a predefined reaction time.

U.S. Pat. No. 5,073,231 discloses an off-line fluorination process forproducing plastic objects with a smooth surface finish. According tothis patent, smooth surfaces are produced by treating plastic objects atelevated temperatures with a mixture of fluorine and an oxidizing agent.Fluorinated surfaces thus produced are claimed to (1) incorporate verylow fluorine in the plastic (less than 6 μg/cm²) and (2) provide goodbarrier action against non-polar solvents.

Improvements in blow molding processes have also been made since thediscovery of the Dixon, et al. higher temperature blow molding processand these are reported in U.S. Pat. No. 4,830,810; 4,617,077; and4,869,859. The '810 patent discloses a blow molding process forproducing containers comprising inserting the preform or parison intothe mold, injecting inert gas into the parison at a first level ofpressure to expand and conform it to the shape of the mold, raising thepressure to a level above the first level by injecting a fluorinecontaining reactant gas into the parison after pressure tightness hasbeen determined. The reactant gas typically comprises a mixture offluorine and nitrogen with the fluorine concentration being about 1% byvolume; the injection pressures are from 4 to about 10 bar and reactiontimes of about 30 seconds. The process eliminates some of the hazardsassociated with blow molding using a reactive gas to conform the parisonto the mold since pressure tightness at the time of injection of thereaction gas may not have been established. The '077 patent modifies the'810 process in that inflation of the parison in the mold with an inertgas is conducted at high pressure followed by treatment of the interiorof the parison with a fluorine containing gas at substantially lowerpressure than that used for initially expanding the parison or preform.After reaction, the reaction gas is replaced with a flushing and coolinggas at a pressure substantially higher than the pressure of the reactiongas and even higher than the initial injection gas used to preform theparison.

U.S. Pat. No. 4,869,859 discloses a blow molding process for thepreparation of high density polyolefin fuel tanks. The patenteesindicate that severe wrinkling of the thermoplastic occurs attemperatures close to or above the melting point, causing an increase inthe permeation rate of the solvent. According to this patent,fluorination is carried out at temperatures from 50° to 130° C.,preferably 80° to 120° C., and below the melting temperature of thepolymer, in an effort to achieve uniform temperature distribution andfluorination of the interior surface of the material.

U.S. patent application having Ser. No. 07/985,665 filed Dec. 3, 1992discloses a multi-step blow molding process for producing permeationresistant thermoplastic containers. A thermoplastic parison is expandedwithin a closed mold by means of an inert gas, the parison vented andthen treated with a reactive gas containing 0.1 to 1% fluorine by volumewhile the parison is at a temperature above its self supportingtemperature for a time sufficient to effect fluorination of the interiorsurface of the parison. Subsequently, the interior surface of thepre-fluorinated parison is treated with a reactive gas containing atleast twice the initial concentration but not less than about 1%fluorine by volume for a specified time to form the fluorinated parisonwith increased permeation resistance.

SUMMARY OF THE INVENTION

This invention relates to an improvement in an in-line, multi-stepfluorination process for the blow molding of thermoplastic containerssuch as fuel containers and bottles which have excellent solvent barrierproperties. The containers have excellent barrier properties withrespect to liquid and vapor permeation by hydrocarbons, polar liquids,hydrocarbon fuels, and hydrocarbon fuels containing polar liquids suchas alcohols, ethers, amines, carboxylic acids, ketones, etc. Theimprovement in the in-line, multi-step fluorination process forconsistently and reliably producing blow molded thermoplastic articleshaving excellent solvent barrier properties resides in (1) forming aparison of thermoplastic material and expanding it within a closed moldby means of an inert gas for conforming the parison to the shape of themold; (2) establishing and maintaining an oxygen contaminant levelgenerally below about 50 ppm in the case of polyethylene andpolypropylene, i.e., non-oxygen containing polymers, theoxygen-to-carbon ratio (O/C) in the fluorinated layer of the polymer ismaintained at less than about 0.08; (3) fluorinating the parisoninitially with a reactive gas containing from about 0.05 to 0.5%fluorine by volume while the thermoplastic is at a temperature above itsself-supporting temperature for a time sufficient to effect fluorinationof the surface of the parison; (4) further fluorinating thepre-fluorinated parison while at an elevated temperature with a reactivegas containing fluorine in a concentration greater than that used in theinitial fluorination but less than about 2.0%, and (5) evacuating theparison, and purging and recovering it from the mold.

The key to the improvement in the process is the combination ofmulti-step fluorination and the establishment and maintenance of anessentially oxygen free environment during the fluorination steps.Because of the low oxygen concentration, the concentration of fluorinein the reactive gas for the first and subsequent fluorination treatmentsteps is controlled in such a way that it eliminates both under and overfluorination of the inside surface of the parison and provides uniformfluorination of the inside surface of the container. There are severaladvantages associated with the in-line multi-step fluorination processto produce containers having improved barrier properties, and theseinclude.

the ability to form permeation resistant containers having enhancedbarrier properties, particularly with respect to hydrocarbons, polarliquids and hydrocarbons containing polar liquids such as alcohols,ethers, amines, carboxylic acids, ketones, etc.;

the ability to produce permeation resistant containers via an in-lineblow molding process while achieving fluorination at commercialproduction rates;

the ability to consistently and reliably produce high densitypolyethylene fuel containers particularly suited for the automotiveindustry, such containers having reduced permeation associated withhydrocarbon fuels blended with lower alcohols such as methanol, ethanol,ethers such as methyl tertiary butyl ether, ketones, etc.; and,

the ability to produce thin-walled containers utilizing reduced levelsof fluorine in the treatment of reactive gas, thereby reducing the totalcost of fluorinating containers.

DRAWINGS

FIG. 1 is a Scanning Electron Micrograph of a fluorinated fuel tanksurface at 5,000X magnification showing the appearance of an overfluorinated surface of the fuel tank.

FIG. 2 is a Scanning Electron Micrograph of a fluorinated fuel tanksurface at 5,000X magnification showing the appearance of an overfluorinated surface of the fuel tank.

FIG. 3 is a Scanning Electron Micrograph of a fluorinated fuel tanksurface at 5,000X magnification showing the appearance of a properlyfluorinated surface of the fuel tank.

FIG. 4 is a Scanning Electron Micrograph of a fluorinated fuel tanksurface at 5,000X magnification showing the appearance of a properlyfluorinated surface of the fuel tank.

FIG. 5 is a Scanning Electron Micrograph of a fluorinated fuel tanksurface at 5,000X magnification showing the appearance of a properlyfluorinated surface of the fuel tank.

FIG. 6 is a Cross-sectional Scanning Electron Micrograph of afluorinated fuel tank at 15,000X magnification showing thickness of thebarrier layer in a properly fluorinated fuel tank.

DETAILED DESCRIPTION OF THE INVENTION

Automotive standards regarding solvent and vapor permeation ratesassociated with hydrocarbon fuels and particularly hydrocarbon fuelscontaining minor portions of lower alkanols have been established forsome time. Fuel tanks constructed of fluorinated high densitypolyethylene and produced via in-line fluorination processes meetcurrent environmental emission requirements from the automobilemanufacturers. However, such fuel containers do not meet the proposedenvironmental emission requirements, particularly when those hydrocarbonfuels are blended with polar liquids such as lower alkanols, e.g.,methanol, ethanol, ethers such as methyl tertiary butyl ether, ketones,amines and other fuel additives. The California Air Resources Board(CARB) has proposed regulations suggesting that a final permeation rateof less than 0.2 grams per day would be desirable for future fuel tanks.

Blow molding of thermoplastic materials to produce containers of varioussizes, wall thicknesses and shapes is well known. Thermoplastic materialsuch as polymers and copolymers of polystyrene, polyacrylonitrile,polyvinyl chloride and particularly polyolefins such as low density andhigh density polyethylene and polypropylene often are used in producingcontainers and they can be treated via in-line fluorination to enhancetheir solvent barrier properties in accordance with this process. Theprocess is particularly adapted for the fluorination of thick-walledcontainers, e.g., 4 millimeters (mm) and greater, typically 4 to about 6mm high density polyethylene for the fabrication of fuel tanks for theautomotive industry and thin-walled, e.g., 3 mm and less wall thickness,bottle-type containers.

In a typical blow molding process for producing hollow articles orcontainers, a thermoplastic material is heated to a temperature aboveits softening point, formed into a parison and injected into a mold. Theparison is inflated or expanded in its softened or molten state viasufficient pressurization with a gas to conform the parison to thecontour of the mold. In many processes, fluorine-containing gasesinitially are used to inflate and conform the parison to the contour ofthe mold. In recent years, the parison initially has been conformed tothe mold via pressurization with a substantially inert gas, e.g.,nitrogen, helium, or argon, to ensure that a seal is formed, thenfluorinated in an effort to reduce environmental contamination andoccupational hazards.

The in-line, multi-step process for fluorinating fuel tanks andthin-walled bottle type polymer containers for producing containershaving an excellent barrier to hydrocarbon solvents containing polarliquids involves carefully controlling the polymer temperature,concentration of fluorine and oxygen in the reactive gas used duringfluorination of the parison and the contact time of fluorination. Inthis process, the parison is evacuated or purged, inflated and initiallycontacted with a reactive gas containing a low concentration of fluorine(0.05 to 0.5% by volume, and preferably 0.2 to 0.4%), the balancethereof being inert under the reaction conditions, at a temperatureabove the self-supporting temperature of the thermoplastic and for atime sufficient to effect surface fluorination. This usually requiresfrom about 2 to 60 seconds, preferably 5 to 45 seconds. Theself-supporting temperature is defined as the temperature above whichthe parison or container will collapse if removed from the mold. A lowconcentration of fluorine is selected for the initial step because theuse of a high concentration of fluorine while the polymer is at atemperature above the self-supporting temperature is believed to damagethe polymer surface, thereby reducing its barrier properties. Thepre-fluorinated parison is then contacted with a reactive gas containinga relatively high concentration of fluorine (greater than 0.3,preferably greater than 0.7 and up to about 2% by volume) in subsequentsteps to further fluorinate the polymer without causing damage to thepolymer surface. Typically, the fluorine concentration will range from0.7 to 1.6%. Excessive localized heating is believed to be reduced sincemany of the available reactive sites are reacted with fluorine in theinitial treatment and because the surface temperature of thepre-fluorinated parison is reduced on contact with the ambienttemperature gas.

The consistency and reliability of the containers as to barrierresistance are controlled by monitoring and controlling the amount ofoxygen contaminant in the parison during the fluorination process. Theoxygen contaminant has been found to compete with fluorine during thefluorination process carried out at an elevated temperature. It isbelieved the oxygen contaminant changes the effective rate of reactionof fluorine with the polymer material and results in the incorporationof oxygen functionalities in the fluorinated polymer layer. The presenceof oxygen contaminant in the parison results in erratic and inconsistentfluorination of the polymer. For example, low, uncontrolled levels ofoxygen with high fluorine concentrations may result in over fluorinationwhile high oxygen levels with low fluorine concentrations may lead tounder fluorination; the parison can either be over fluorinated or underfluorinated depending on the amount of oxygen contaminant in the parisonand the concentration of fluorine used during fluorination. The under orover fluorination of the parison as well as the incorporation of oxygenfunctionalities in the fluorinated layer decreases the solventpermeation resistance of the containers.

A key feature of the process involves carefully purging the parison andinert gas and reactive gas feed lines to reduce the oxygen contaminantlevel below about 50 ppm in the parison prior to fluorination. Bykeeping the oxygen concentration low, the desired O/C ratio in thefluorinated layer is maintained. However, because of the difficulty inanalyzing oxygen levels during the fluorination process steps, it iseasier to verify the effectiveness of the purging process for theremoval of oxygen at each gas feed and exhaust line to the polymercontainer by analyzing the exhaust gas from a purged nonfluorinatedparison or by analyzing the fluorinated surface of containers withrespect to the oxygen/carbon ratio. If the ratio of oxygen to carbon inthe fluorinated layer is 0.08 or above, then the process may requireadditional purging of the gas feed lines. (Monitoring of oxygen is donein the absence of fluorine because of the corrosivity of the fluorine toanalytical equipment. It is the effectiveness of the purge with inertgas that is controlling and its effectiveness can be measured in theabsence of fluorine.) With a reduction in oxygen contamination, theconcentration of fluorine in the reactive gas for the first andsubsequent fluorination treatment steps can be reduced vis-a-visfluorination in an oxygen contaminated atmosphere. It can be controlledin such a way that it eliminates both under and over fluorination of theinside surface of the treated parison.

The in-line, multi-step fluorination process utilizes an extremelydilute fluorine containing gas, e.g., greater than 0.05%, but generallynot greater than about 0.5%, and preferably between 0.2 and 0.4% byvolume as the fluorinating agent in the initial fluorination ofthermoplastic containers while carefully minimizing and/or eliminatingthe oxygen contaminant in the container and while the thermoplastic isat a temperature above the self-supporting temperature of the polymer.For example, a temperature generally varying from about and above 105°C. to 130° C. would be required for high density polyethylene (HDPE). Ifhigher concentrations of fluorine in the reactant gas, e.g., greaterthan 0.5%, are injected into the parison while carefully minimizingand/or eliminating the oxygen contaminant in the parison and while thepolymer is above the self-supporting temperature, the fluorine willreact aggressively with the polymer and will cause over fluorinationresulting in increased solvent permeation. The contact or reaction timefor the initial fluorination step may range from about 2 to 60 seconds,preferably from about 5 to 45 seconds. Although contact times can extendfor very dilute reactive gas streams beyond 60 seconds, no significantadvantages are expected to be achieved. However, if a reaction time ofless than 2 seconds is employed for the initial fluorination, thefluorine will not have enough time to react with the polymer and thepolymer will be under fluorinated. Reaction or contact pressures for theinitial fluorination step are conventional and range from 2 to 50 bar.

Since the initial fluorine treatment is carried out above theself-supporting temperature of the polymer, it is very important tocarefully control the time involved in expanding and conforming theparison to the mold with an inert gas, if such initial step is used. Thetemperature of the polymer surface is largely dependent on the thicknessof the wall, the temperature of the inflating gas, and the contact timenecessary for expanding and conforming the parison to the mold. Forexample, thin-walled containers cool rapidly on contact with roomtemperature inflation gas, thereby permitting the use of relativelyhigher fluorine concentrations in the reactive gas. Thick-walledcontainers maintain elevated temperature for a longer period of time andrelatively lower fluorine concentrations in the reactive gases are oftenrequired because of excessive surface temperatures. In any case, it isimportant to carefully control the expanding and conforming time in sucha way that the initial fluorine treatment with a reactive gas containingan extremely low concentration of fluorine is carried out at atemperature above the self-supporting temperature of the polymer.

Once the surface of the polymer has been fluorinated with a lowconcentration of fluorine in the initial step at a temperature above theself-supporting temperature of the polymer, further fluorination of thesurface to achieve the final reduction in permeability and enhancedphysical properties is effected. Secondary fluorination of theprefluorinated parison is carried out by contacting the surface with afluorine-containing gas with the fluorine concentration ranging fromabout 0.3 to about 24%, preferably from about 0.7 to about 1.6% byvolume but the concentration of fluorine typically is from about 11/2 to4 times that used in the initial fluorination step. It is, once again,important to carefully minimize and/or eliminate the oxygen contaminantin the parison during fluorination to obtain fuel tanks with consistentand reliable solvent permeation resistance. If the secondaryfluorination is carried out with higher than 2% fluorine in the reactivegas with low levels of oxygen contaminant in the parison while thepolymer is at elevated temperatures, the fluorine can react aggressivelywith the polymer and cause over fluorination. The secondary fluorinationof the prefluorinated parison is carried out for a period of 2 to 60seconds, preferably a period of 5 to 45 seconds when the container walltemperature is at an elevated temperature but below the self-supportingtemperature. Once again, if a reaction time of less than 2 seconds isemployed for the secondary fluorination, the fluorine may not haveenough time to react with the polymer and the polymer will be underfluorinated. It is treatment of the pre-fluorinated parison with ahigher concentration of fluorine in the subsequent reactive gas in thepresence of the minimum amount of oxygen contaminant that causes asecond fluorination of the parison to occur, thereby creating a secondfluorine-polymer gradient. With each secondary fluorine treatment step,enhanced fluorination is achieved and essentially no over fluorinationis observed by the reaction of reactive gas having a slightly higherconcentration of fluorine gas with the thermoplastic polymer.

In a preferred embodiment, containers with excellent solvent permeationresistance are made consistently and reliably from polyethylene. Initialfluorination of the parison, and preferably at least the first treatmentassociated with the secondary fluorination, is to be conducted at abovethe self-supporting temperature of polyethylene while carefullyminimizing and/or eliminating the oxygen contaminant in the parison. Forpolyethylene, the self-supporting temperature will vary from 105° C. to130° C. and with final temperatures reaching below about 100° C. priorto ejection of the parison from the mold. If more than two fluorinationsteps are used in the multi-step process, subsequent steps may becarried out at temperatures below the self-supporting temperature ofpolyethylene.

The concentration of fluorine in the reactive gas for the fluorinationsteps is selected in such a way that it eliminates both under and overfluorination of the polymer. The polymer is considered to be underfluorinated if the fluorinated layer contains a fluorine concentrationof less than about 10 μg/cm², preferably less than about 15 μg/cm², asdetermined by a combination of x-ray fluorescence (XRF) and Rutherfordback scattering (RBS) techniques. A lower concentration of fluorine inthe barrier layer would result in poor solvent permeation resistance.

Although an effective fluorination treatment results in some "rumpling"of the polymer surface, an over fluorinated surface displays a morepronounced rumpling, the magnitude of which can be quantified by theincrease in the surface area. It is believed that the higher solventpermeation rate observed is due to an increase in surface area availablefor transport, as well as possible surface stresses induced by therumpling. The polymer is considered to be over fluorinated if thefluorinated layer has a ratio of actual surface area to geometricsurface area greater than about 2, preferably greater than about 1.8.The actual and geometric surface areas are determined by using scanningtunneling microscopy (STM). The geometric surface area corresponds tothe surface area of a perfectly flat surface.

To summarize, in the improved in-line, multi-stage process, initialcontact of the parison is made with a very dilute fluorine containingreactive gas for fluorinating the surface of the parison while thethermoplastic is in its softened state and at a temperature above theself-supporting temperature for limited times. Oxygen contaminant levelsare maintained at low levels, e.g., below 50 ppm or such that anoxygen-to-carbon ratio in the fluorinated polymer layer is below 0.08.The very dilute fluorine-containing reactive gas will have a fluorineconcentration not greater than about 0.54 and preferably between 0.2 and0.4% by volume. Reaction periods of about 2 to 60 seconds, preferablyfrom about 5 to 45 seconds, are employed. Initial surface fluorinationat these concentrations effects an initial fluorinated polymer gradient.Secondary fluorination requires a reactant gas having a slightly higherfluorine concentration, typically from about 0.3 to about 2% andpreferably from about 0.7 to 1.6% while carefully minimizing and/oreliminating the oxygen contaminant in the parison. Contact times, onceagain, range from 2 to 60 seconds, preferably from 5 to 45 seconds forsecondary treatment steps. If the duration of the fluorination step istoo short or the oxygen contaminant present in the parison is too high,the polymer will be under fluorinated and will have a surface fluorineconcentration of less than about 10 μg/cm², preferably less than about15 μg/cm². Likewise, if the concentration of fluorine present duringfluorination in the absence of the oxygen contaminant is too high, thepolymer surface will be over fluorinated and will have a ratio of actualsurface area to geometric surface area greater than about 2, preferablygreater than about 1.8. In either case the fluorinated container willdisplay poor solvent permeation resistance.

The following examples are provided to illustrate various embodiments ofthe invention and are not intended to restrict the scope thereof.

CONTROL EXAMPLE 1

Automobile fuel tanks with a nominal wall thickness of 4.0 mm wereprepared from a high density polyethylene (HDPE) by the extrusion blowmolding process. During the inflation process the fuel tanks werepressurized to approximately 100 psig with an inert nitrogen gas for aperiod of 9 seconds (Step 1 or Gas 1), after which the pressure wasreleased from the fuel tanks to allow the nitrogen gas to escape.Because of relatively thick walls of the fuel tanks, the temperature ofthe polymer remained above the self-supporting temperature of HDPE. Thefuel tanks then were repressurized a second time to approximately 100psig with a reactive treatment gas containing 0.5% fluorine in inertnitrogen for a period of 9 seconds (Step 2 or Gas 2). After this thepressure was released from the fuel tanks to allow reactive gas toescape. The fuel tanks were pressurized a third time to approximately100 psig with a reactive treatment gas containing 2.5% fluorine in inertnitrogen for a period of 9 seconds. Finally, the fuel tanks were ventedto atmospheric pressure, purged with an ambient pressure air stream oran inert gas to remove any traces of reactive treatment gas and removedfrom the mold. The tanks were, therefore, treated first with a fluorinecontaining gas at above the self-supporting temperature of HDPE. Theywere treated further with a fluorine containing gas at a temperatureabove or below the self-supporting temperature of HDPE or both above andbelow.

A fuel tank prepared in the above manner was filled with a hydrocarbonsolvent mixture consisting of 92.5% indolene (a mixture of hydrocarbonswhich simulates an unleaded automotive fuel), 5.0% methanol, and 2.5%ethanol by volume to determine hydrocarbon solvent permeability. Themouth of the filled fuel tank was then heat sealed with foil-backed lowdensity polyethylene film and capped. The sealed fuel tank was stored inan ambient pressure air circulating oven at 40° C. for 6 weeks, and itshydrocarbon permeation (weight loss) was monitored periodically. Thisfuel tank showed a hydrocarbon solvent permeability value of about 0.08grams per day which was well below the value proposed by CARB. Althoughthe values obtained are good, the results obtained were perhapsfortuitous as Examples 2 and 3 show. Better and more consistent resultscan be obtained with better oxygen control through monitoring andpurging of the lines and parison as is set forth in later examples.

CONTROL EXAMPLE 2

The procedure for fluorinating high density polyethylene (HDPE) fueltanks described in Control Example I was repeated using the identicalconditions. The hydrocarbon solvent permeability one of these fuel tankswas determined following the same procedure as described in ControlExample 1. This tank showed a hydrocarbon solvent permeability valueclose to 0.43 grams per day which was considerably higher than noted inControl Example 1 and specified by CARB.

CONTROL EXAMPLE 3

The procedure for fluorinating high density polyethylene (HDPE) fueltanks described in Control Example i was repeated again using theidentical conditions. The hydrocarbon solvent permeability of one ofthese fuel tanks was determined following the same procedure asdescribed in Control Example 1. This tank showed a hydrocarbon solventpermeability value close to 0.47 grams per day which was considerablyhigher than noted in Control Example 1 and specified by CARB.

One of the fluorinated fuel tanks was analyzed to determine thethickness and composition of the fluorinated layer. The cross-sectionalanalysis using scanning electron microscopy (SEM) revealed a barrierthickness of ˜0.2 μm. The surface composition determined by electronspectroscopy for chemical analysis (ESCA) revealed an O/C ratio of ˜0.08in the top ˜100 Å of the barrier layer, indicating a significantincorporation of oxygen in the barrier layer.

CONTROL EXAMPLE 4 HIGH FLUORINE LEVELS

The two step fluorination process described in Control Example 1 wasrepeated using a similar procedure with the exception of employing 0.8%and 5.0% fluorine in the first and second fluorination steps,respectively. However, precautions were taken including purging theparison and gas feed lines with inert nitrogen gas to minimize and/oreliminate oxygen contaminant prior to and during fluorination of fueltanks. The level of oxygen contaminant in feed and exhaust lines to thepolymer container was measured to be below 50 ppm. The fluorinatedbarrier thickness determined by analyzing one of the tanks was ˜0.3 μm.The O/C ratio in the barrier layer was ˜0.01, indicating considerablereduction in the incorporation of oxygen in the barrier layer bycontrolling the level of oxygen contaminant in the feed and exhaustlines to the polymer container.

The hydrocarbon solvent permeability of two of these fuel tanks wasdetermined following a procedure similar to the one described in ControlExample 1. These tanks showed a hydrocarbon solvent permeability valueabove 0.5 grams per day which was higher than noted in Control Examples1 to 3 and specified by CARB.

The poor solvent permeation resistance value was believed to be relatedto over fluorination of the fuel tanks which was caused by the use ofhigh concentration of fluorine during the first and second steps in theabsence of significant oxygen contamination. Over fluorination of thefuel tanks produced an extremely rough fluorinated surface layer, asshown by a scanning electron micrograph (SEM) in FIG. 1. Furthermore,the over fluorination of the fuel tanks was evidenced by the high ratioof the actual surface area of the fluorinated layer to the geometricsurface area-it was greater than about 2.0.

CONTROL EXAMPLE 5 LOW AND HIGH FLUORINE LEVELS

The two step fluorination process described in Control Example I wasrepeated using a similar procedure with the exception of employing 0.25%and 2.5% fluorine in the first and second fluorination steps,respectively. Further precautions were taken vis-a-vis Control Example 4including purging the parison and gas feed lines with inert nitrogen gasto minimize and/or eliminate oxygen contaminant prior to and duringfluorination of fuel tanks. The concentration of fluorine used in thefirst and second steps of this example was considerably lower than usedin Control Example 4. The level of oxygen contaminant in the feed andexhaust lines to the polymer container was measured to be below 50 ppm.The thickness of the fluorinated barrier layer was ˜0.3 μm. The O/Cratio in the barrier layer was ˜0.01.

The hydrocarbon solvent permeability of six of these fuel tanks wasdetermined following the procedure similar to the one described inControl Example 1. These tanks also showed a hydrocarbon solventpermeability value above 0.5 grams per day which was higher than notedin Control Examples 1 to 4 and specified by CARB.

It is believed the poor solvent permeation resistance value was onceagain related to over fluorination of the fuel tanks caused by the useof high concentration of fluorine during the second step in the absenceof significant oxygen contamination. Over fluorination of the fuel tanksproduced an extremely rough fluorinated surface layer, as shown by ascanning electron micrograph in FIG. 2. Furthermore, the overfluorination of the fuel tanks was evidenced by the high ratio of theactual surface area of the fluorinated layer to the geometric surfacearea; it was greater than about 2.0.

CONTROL EXAMPLE 6 LOW FLUORINE LEVEL, SINGLE-STEP PROCESS

Automobile fuel tanks with a nominal wall thickness of 4.0 mm wereprepared from a high density polyethylene (HDPE) by the extrusion blowmolding process. During the inflation process the fuel tanks werepressurized to approximately 100 psig with an inert nitrogen gas forapproximately 5 seconds, after which the pressure was released from thefuel tanks to allow the nitrogen to escape. Because of the relativelythick walls of the fuel tanks, the temperature of the polymer remainedabove the self-supporting temperature of HDPE. The fuel tanks then wererepressurized a second time to approximately 100 psig with a reactivetreatment gas containing 0.25% fluorine in inert nitrogen for a periodof 16 seconds. After this the pressure was released from the fuel tanksto allow the reactive gas to escape. Finally, the fuel tanks were ventedto atmospheric pressure, purged with an ambient pressure air stream oran inert gas to remove any traces of reactive treatment gas and removedfrom the mold. Proper precautions were taken including purging theparison and gas feed lines with inert nitrogen gas to minimize and/oreliminate oxygen contaminant prior to and during fluorination of thefuel tanks. The fluorination of these tanks was initiated at above theself-supporting temperature of HDPE. In summary, these tanks wereprepared using a single step treatment with low concentration fluorinegas. The thickness of the barrier layer was less than about 0.1 μm. TheO/C ratio in the barrier layer was ˜0.03.

The hydrocarbon solvent permeability of two of these fuel tanks wasdetermined following a procedure similar to the one described in ControlExample 1. The tanks showed a hydrocarbon solvent permeability above 0.5g/day, which was considerably higher than specified by CARB.

The poor solvent permeation resistance value was related to underfluorination of the polymer surface. This under fluorination wasevidenced by a fluorine content in the barrier layer of approximately 8μg/cm² as determined by a combination of x-ray fluorescence (XRF) andRutherford back scattering (RBS) techniques. This example also showsthat it is not possible to produce containers with enhanced solventpermeation resistance by simply using a one-step process. It also showsthat a fluorine content in the barrier layer substantially greater than8 μg/cm² is required to produce fuel tanks with good solvent permeationresistance.

EXAMPLE 7 LOW-LOW FLUORINE LEVELS

Automobile fuel tanks with a nominal wall thickness of 4.0 mm wereprepared from a high density polyethylene (HDPE) by the extrusion blowmolding process. During the inflation process the fuel tanks werepressurized to approximately 100 psig with an inert nitrogen gas for aperiod of 5 seconds (Step 1 or Gas 1), after which the pressure wasreleased from the fuel tanks to allow the nitrogen gas to escape.Because of relatively thick walls of the fuel tanks, the temperature ofthe polymer remained above the self-supporting temperature of HDPE. Thefuel tanks then were repressurized a second time to approximately 100psig with a reactive treatment gas containing 0.25% fluorine in inertnitrogen for a period of 15 seconds (Step 2 or Gas 2). After this thepressure was released from the fuel tanks to a low reactive gas toescape. The fuel tanks were pressurized a third time to approximately100 psig with a reactive treatment gas containing 0.7% fluorine in inertnitrogen for a period of 10 seconds (Step 3 or Gas 3). After this thepressure was released from the fuel tanks to allow reactive gas toescape. The fuel tanks were pressurized a fourth time to approximately100 psig with a reactive treatment gas containing 1.25% fluorine ininert nitrogen for a period of 10 seconds (Step 4 or Gas 4). Finally,the fuel tanks were vented to atmospheric pressure, purged with anambient pressure air stream or an inert gas to remove any traces ofreactive treatment gas and removed from the mold. Precautions were takenincluding purging the parison and gas feed lines with inert nitrogen gasto minimize and/or eliminate oxygen contaminant prior to and duringfluorination of fuel tanks. The level of oxygen contaminant in the feedand exhaust lines to the polymer container was measured to be less than50 ppm. Furthermore, the concentration of fluorine for the first andsubsequent fluorination steps was selected carefully to eliminate bothunder and over fluorination of the fuel tanks. The tanks were,therefore, treated first with a fluorine containing gas at above theself-supporting temperature of HDPE. They were treated further with afluorine containing gas at a temperature above or below theself-supporting temperature of HDPE or both above and below. Thethickness of barrier layer was ˜0.3 μm. The O/C ratio in the barrierlayer was ˜0.01.

The hydrocarbon solvent permeability of two of these fuel tanks wasdetermined following the procedure similar to the one described inControl Example 1. These tanks showed a hydrocarbon solvent permeabilityvalue close to 0.03 grams per day which was considerably lower thannoted in Control Example I and specified by CARB. The ratio of theactual surface area of the fluorinated layer to the geometric surfacearea was approximately 1.5, indicating that the interior surface of thefuel tanks was not over fluorinated. Additionally, the fluorinated layerproduced in this example had a considerably lower surface roughness thanControl Examples 4 and 5, as shown by a scanning electron micrograph inFIG. 3. The concentration of fluorine in the fluorinated layer wasapproximately 60 μg/cm², indicating that the interior surface of thefuel tanks was not under fluorinated. From these data, surfacefluorination can range from about 15 to 80 g/cm². However, it isbelieved that fluorination can be as high as possible, e.g., ≧100 μg solong as the ratio of actual surface area to geometric surface area ismaintained.

EXAMPLE 8 LOW-LOW FLUORINE LEVELS

The procedure for fluorinating high density polyethylene (HDPE) fueltanks described in Example 7 was repeated using the identicalconditions. The hydrocarbon solvent permeability of three of these fueltanks was determined following the procedure similar to the onedescribed in Control Example 1. These tanks showed a hydrocarbon solventpermeability value close to 0.05 grams per day which was, once again,considerably lower than noted in Control Example 1 and specified byCARB, but consistent with Example 7. The ratio of the actual surfacearea of the fluorinated layer to the geometric surface area wasapproximately 1.4, indicating that the interior surface of the fueltanks was not over fluorinated. The concentration of fluorine in thefluorinated layer was approximately 50 μg/cm², indicating that theinterior surface of the fuel tanks was not under fluorinated. The O/Cratio in the barrier layer was ˜0.02.

EXAMPLE 9 LOW-LOW FLUORINE LEVELS

The procedure for fluorinating high density polyethylene (HDPE) fueltanks described in Example 7 was repeated using the identicalconditions. The hydrocarbon solvent permeability of six of these fueltanks was determined following the procedure similar to the onedescribed in Control Example 1. These tanks showed a hydrocarbon solventpermeability values ranging from 0.06 to 0.09 grams per day which were,once again, lower than noted in Control Example 1 and specified by CARBand consistent with Examples 7 and 8. The ratio of the actual surfacearea of the fluorinated layer to the geometric surface area wasapproximately 1.4, indicating that the interior surface of the fueltanks was not over fluorinated. Additionally, the fluorinated layersproduced in this example revealed considerably lower surface roughnessthan Control Examples 4 and 5, as shown by scanning electron micrographsin FIGS. 4 and 5. The concentration of fluorine in the fluorinated layerwas approximately 50 μg/cm², indicating that the interior surface of thefuel tanks was not under fluorinated. The O/C ratio in the barrier layerwas ˜0.02.

EXAMPLE 10 LOW-LOW FLUORINE LEVELS

The procedure for fluorinating high density polyethylene (HDPE) fueltanks described in Example 7 was repeated using the identicalconditions. The hydrocarbon solvent permeability of seven of these fueltanks was determined following the procedure similar to the onedescribed in Control Example 1 with the exception of using pure indolenesolvent. These tanks showed a hydrocarbon solvent permeability valueranging from 0.01 to 0.03 grams per day, indicating an excellentpermeation barrier to nonpolar hydrocarbon solvents. The thickness ofbarrier layer was ˜0.3 μm, as can be seen in FIG. 6. The O/C ratio inthe barrier layer was ˜0.01.

SUMMARY

The above Control Examples clearly show that it is difficult to producefuel tanks with enhanced solvent permeation resistance consistently andreproducibly with unmonitored and uncontrolled oxygen contamination inthe parison during the fluorination process. Also, the above Examples 4and 5 revealed that the consistency and reliability problems can not beresolved simply by minimizing and/or eliminating oxygen contaminant fromthe parison. They also showed that these problems can not be resolved bymanipulating the concentration of fluorine in the first step alone. Theconsistency and reliability problems are solved by controlling the rateof fluorination in the first step and in all subsequent steps, whilecarefully minimizing and/or eliminating oxygen contamination.

What is claimed is:
 1. In a process for the production of a blow molded,permeation resistant thermoplastic article via in-line fluorinationwherein a parison of thermoplastic material is formed, expanded within aclosed mold by means of an inert inflation gas for conforming theparison to the shape of the mold and fluorinated under conditionssufficient to effect surface fluorination of the interior of saidparison thereby reducing its permeability to hydrocarbon fuels,evacuated and the article recovered, the improvement which resides in anin-line multi-step fluorination process which comprises thesteps:establishing and maintaining an oxygen concentration of less thanabout 50 ppm by volume in the parison; fluorinating the parison while ata temperature above its self-supporting temperature with a reactivefluorine containing gas containing from about 0.05 to 0.5% fluorine byvolume for a time sufficient to effect initial fluorination of theinterior surface of the parison and thereby form a pre-fluorinatedparison; and then, subsequently fluorinating the interior surface of thepre-fluorinated parison for effecting secondary fluorination bycontacting the interior surface with a reactive fluorine containing gashaving a fluorine concentration higher than that employed in the initialfluorination but not greater than about 2% by volume, saidpre-fluorinated parison being at an elevated temperature but at atemperature below the self-supporting temperature during at least partof the secondary fluorination.
 2. The process of claim 1 wherein theparison is a thick-walled parison having a nominal thickness of about 4mm and greater.
 3. The process of claim 1 wherein the thermoplasticmaterial is high density polyethylene.
 4. The process of claim 3 whereinthe pressure during pressurization to form the pre-fluorinated parisonis from 2 to 50 bar.
 5. The process of claim 4 wherein the concentrationof fluorine in the reactive gas used for initial fluorination of theparison is from 0.2 to 0.4 percent by volume.
 6. The process of claim 5wherein the contact time for the initial fluorination is from about 2 to60 seconds.
 7. The process of claim 6 wherein the concentration offluorine in the reactive gas used for secondary fluorination is fromabout 0.3 to 2 percent by volume.
 8. The process of claim 7 wherein thecontact time in the secondary fluorination is from 2 to 60 seconds. 9.The process of claim 8 wherein the concentration of fluorine in thereactant gas used for secondary fluorination is from 0.7 to 1.6 percentby volume.
 10. The process of claim 9 wherein the parison is formed,expanded, conformed to the shape of the mold with an inert gas selectedfrom the group consisting of nitrogen, helium and argon and the oxygenconcentration of the gas within the parison monitored.
 11. In a processfor the production of a blow molded, permeation resistant thermoplasticarticle via in-line fluorination wherein a parison of thermoplasticmaterial selected from the group consisting of polyethylene andpolypropylene is formed, expanded within a closed mold by means of aninert inflation gas for conforming the parison to the shape of the moldand fluorinated under conditions sufficient to effect surfacefluorination of the interior of said parison thereby reducing itspermeability to hydrocarbon fuels, evacuated and the article recovered,the improvement which resides in an in-line multi-step fluorinationprocess which comprises the steps:fluorinating the parison while at atemperature above its self-supporting temperature with a reactivefluorine containing gas containing from about 0.05 to 0.5% fluorine byvolume for a time sufficient to effect initial fluorination of theinterior surface of the parison and thereby form a pre-fluorinatedparison; and then, subsequently fluorinating the interior surface of thepre-fluorinated parison for effecting secondary fluorination bycontacting the interior surface with a reactive fluorine containing gashaving a fluorine concentration higher than that employed in the initialfluorination but not greater than about 2% by volume, saidpre-fluorinated parison being at an elevated temperature but at atemperature below the self-supporting temperature during at least partof the secondary fluorination; establishing and maintaining an oxygenconcentration during the fluorination of the parison such that thepermeation resistant article has an oxygen to carbon ration of less than0.08 in the fluorinated layer.
 12. The process of claim 11 wherein thepressure during pressurization to form the pre-fluorinated parison isfrom 2 to 50 bar.
 13. The process of claim 11 wherein the concentrationof fluorine in the reactive gas used for initial fluorination of theparison is from 0.2 to 0.4 percent by volume.
 14. The process of claim13 wherein the contact time for the initial fluorination is from 2 to 60seconds.
 15. The process of claim 14 wherein the thermoplastic is highdensity polyethylene.
 16. The process of claim 15 wherein theconcentration of fluorine in the reactive gas used for secondaryfluorination is from about 0.7 to 1.6 percent by volume.
 17. The processof claim 16 wherein the contact time in the second fluorination is from2 to 60 seconds.